We have a Moby so it may be different but we stopped when he started pushing off with his feet. He was acting like he wanted more freedom/space so I switched to letting his legs dangle and he's been much happier. Your baby will tell you when it's time. FYI - It was around 2 months for us.
With a mei tai, I'd still froggy leg LO at two months. I think they are more ready to straddle after 4 months. We have a Moby and at two month we switched to an upright cradle hold where DD's head was right up against my shoulder. I never put her low enough to be breathing issue.
Re: at what age did you stop doing the newborn hold in carriers?